My computers are running Windows 7 and 8, respectively, and the latest update of Firefox. I've read a few Mozilla background articles on this issue, but it's hard to believe that every single website -- including Google, Netflix, Amazon, major newspapers, banks, etc. -- have deficient certificates. Sometimes I'm allowed to bypass the warning and whitelist the site as an exception (which is inconvenient); other times this option isn't permitted, so I switch over to Chrome (which never seems to regard the site as unsafe). This is like the boy who cried wolf: When users are trained to automatically whitelist all sites, we're not going to block the occasional website that actually IS unsafe. In any case, this has been going on so long and is so annoying that I'm close to pulling the plug on Firefox altogether. And I'm sure I'm not alone.

P.S. Someone on another forum suggested deleting/renaming FF's cert8 database file. All that did was wipe out my whitelist.